Agreed. Unless the defense is supremely confident they can have the confession suppressed, pleading "not guilty" is not a good move.
This is a comment from Lancasteronline
"Ludwig's plea of not guilty is a "procedural formality," meaning that this is a common practice. It also says that the plea doesn't mean that's what he's going to plead when he goes before a judge, nor does it even mean there's going to be a trial."
Seems like a lot of things in PA are different. And, he has already admitted guilt. Seems odd to me.